Modern vehicles are equipped with an interior rear-view mirror which, in use, is positioned to allow the driver to view other vehicles behind his or her vehicle. During night time driving conditions, glare caused by the reflection of following vehicle headlights in these rear-view mirrors presents a problem to the driver. There is a need to reduce this glare while allowing the driver to continue to see behind the vehicle.
Known solutions to night time headlight glare have included interior rear-view mirrors that are manually manipulable between a daytime reflecting position and a night-time reflecting position. These mirrors use first and second surface reflections from prisms with a reflective coating on one surface.
Window panes are common in buildings and vehicles and usually permit light and other radiation to enter an enclosed space such as a room or the interior of a vehicle, etc. Often the intensity of light, particularly sunlight, can cause a problem if the light is very bright or the radiation heats the interior or fades materials in the interior of the room or vehicle. There is occasionally a need to reduce the intensity of light transmitted into a room or vehicle interior, etc, whilst still permitting some light to enter. Window glare can be reduced by tinting the windows, however, tinting permanently reduces the window's light transmissivity.
Other window panes may be used inside a building or vehicle to partition off an area. For example a window may partition the driver of a vehicle from his rear passengers or a window may partition a private office area from a public space. There is occasionally a need to decrease the transparency of such a window pane to increase the level of privacy to the area. Again, window tinting is unsatisfactory since there is a permanent reduction in transparency where reversibility of transparency would be preferred.
It is known that electrochromics can be used to reduce the intensity of reflected light from a vehicle rear-view mirror or, alternatively, to reduce the transmissivity of light through a window pane. An electrochrome is an inorganic or organic substance whose absorption characteristics are altered by charge or electron transfer when subjected to an applied voltage potential. This change in absorption usually gives rise to a variation in the electrochrome's colour or transparency. An electrochromic assembly typically has two electrodes and an electrochrome positioned such that it can be affected by a voltage potential applied across the electrodes which promotes charge transfer to the electrochrome. By appropriately varying the voltage potential the electrochrome's transmission characteristics can be varied. Clearly this effect can be utilised advantageously in the fabrication of windows and mirrors.
A problem with known electrochromic assemblies that use conventional liquid electrolytes is that the life-time of the assembly is relatively short compared with the life-time of the vehicle mirror or window pane. The short life span of the electrochromic assembly is attributed to the electrochemical degradation by over oxidation or over reduction of the electrochrome and/or electrodes which degrades the assembly. Replacement of the electrochromic assembly is costly and inconvenient to the user.
A further problem with known electrochromic assemblies containing liquid electrolytes is that liquid may leak from the assembly and damage the surrounding environment. This leakage may occur during normal operation or following impact e.g. during an accident. A further disadvantage of assemblies containing liquid electrolytes is that it is difficult to reduce the profile (or thickness) of the liquid containing assembly since a separator is required to separate the two electrodes so that they do not contact one another. Usually the separator is a gasket around the border of the assembly, however, it can be difficult to maintain a uniform gap between the electrodes and external or internal stresses can vary the gap between the electrodes resulting in non-uniform performance due to disruption of electric fields. This is evident in LCD displays where pressure applied externally results in discolouration of the display over a large area. This becomes especially difficult when trying to construct electrochromic assemblies which are physically large, curved within either a single plane or across two planes. This additional complexity adds to cost and manufacturability.
Another problem with known electrochromic assemblies is that the materials from which they are constructed are often hazardous to health and the environment. For example viologens are popularly used as electrochromics however viologens are toxic in contact with skin and if swallowed. Viologens also irritate the respiratory system and the skin and are toxic to aquatic organisms and may cause long-term adverse effects in the aquatic environment.
One alternative to a liquid electrolyte is a solid electrolyte. An electrochromic assembly having a solid electrolyte usually has a much longer life span than an assembly containing only a liquid electrolyte. A disadvantage of solid electrolytes is that the electrochromic layer and the solid electrolyte usually have rigid structures which must continuously expand and contract to allow ions to enter and leave the electrochromic layer. Ultimately mechanical strains develop at the interface of the electrochromic layer and the solid electrolyte resulting in overall degradation of the electrochromic assembly. In particular such assemblies exhibit a hazing or crazing due to light scattered by cracks and the colour or transparency change of the assembly is also diminished due to the resultant impeded ion flow.
Furthermore, known electrochromic assemblies can be difficult and/or costly to assemble during manufacturing which is inconvenient for efficient, cost-effective production.
Accordingly there is a need for a charge conducting medium suitable for use in an electrochromic assembly, particularly where rapid changes in the colour, transparency or reflectivity of the associated electrochrome are required.
It is an object of the present invention to provide an improved electrochromic assembly which eliminates or at least reduces some of the problems associated with the prior art discussed above.
The invention will be generally discussed in relation to dimming mirrors for use in automobiles but it is not so restricted and may be applied to other systems where a decrease in reflectivity or transmissivity of light or other radiation is required, for example only, and not limited to electrochromic glazing, ophthalmic applications, visual displays or clothing.